Obesity and diabetes affect quality of life of a large portion of the national population and has a significant economic impact on healthcare. Understanding the precise molecular events that lead to insulin resistance and metabolic syndrome will translate into more effective treatments. We have shown that dietary methionine restriction (MR) produces a coordinated series of physiological responses that include increased energy expenditure (EE), reduced fat deposition, and a profound enhancement of insulin sensitivity. The metabolic effects of dietary MR are accompanied by transcriptional responses in liver and adipose tissue that reduce circulating and tissue lipids. Our published and preliminary data provide compelling evidence that dietary MR increases UCP1 expression in both brown and white adipose tissue through increases in sympathetic nervous system (SNS) activity, but it has not been established whether increased UCP1 expression or function is an essential mediator of one or more of the physiological responses to dietary MR. Thus, a key objective of our proposal is to determine the significance of modulation of UCP1 by dietary MR in producing the resulting metabolic phenotype. A rigorous evaluation of substrate switching in peripheral tissues during the transition from fasting to the fed state, coupled with in vivo measures of de novo lipogenesis, indicates that dietary MR enhances energetically inefficient conversion of glucose to lipid in the fed state when rates of EE are highest. Cold exposure produces a similar induction of inefficient glucose-dependent lipogenesis in adipose tissue to support the simultaneous increase in fatty acid oxidation that occurs during cold exposure. This response also occurs in cold-exposed mice lacking UCP1, suggesting that increased SNS-dependent, but UCP1-independent substrate cycling is an important component of the thermo genic response. We provide compelling new data showing that UCP1 plays an essential, previously unappreciated role in regulating lipid cycling between liver and adipose tissue in a manner that impacts in vivo insulin sensitivity. Thus, our overall hypothesis is that UCP1 plays an essential role in the integration of lipid metabolism between liver and adipose tissue, and the remodeling of peripheral lipid metabolism and increase in insulin sensitivity produced by dietary MR utilizes SNS-dependent regulation of UCP1 to affect both metabolic responses. Accordingly, our Specific Aims will test the hypotheses that 1) the ability of dietary MR to remodel the integration of lipid metabolism between liver and adipose tissue is regulated by and dependent upon UCP1, and 2) UCP1 is essential and necessary for dietary MR to enhance insulin sensitivity. In Aim 1, we will use UCP1-/- mice to determine whether UCP1 is necessary for dietary MR to increase EE and reciprocally remodel lipid metabolism in adipose tissue and liver. In Aim 2, we will employ a similar loss of function approach to determine the role of UCP1 in the enhancement of overall and tissue-specific insulin sensitivity by dietary MR. The results of the proposed studies will provide important new mechanistic insights into how dietary MR affects lipid metabolism and energy balance.